Current steering systems for road vehicles possess a mechanical link between the vehicle steering wheel, or hand wheel actuator, and the steered road wheels of the vehicle. Future steering systems may replace the mechanical link by an electro-hydraulic link or a mechanical-electronic link. These future systems, which do not use the mechanical link, may be generally referred to as steer-by-wire systems. In a steer-by-wire system, some mechanism is provided to steer the road wheels. Steering of the road wheels in a steer-by-wire system can be accomplished with appropriate electric motors and power electronics. Multiple motors at the road wheels provide a level of redundancy to maintain the capability to steer when a failure occurs in a single motor. However, a disadvantage with such systems is that when all of the electric motors at the road wheels fail, then the steering function can no longer be performed by the steer-by-wire system. Additionally, since a conventional mechanical link is not provided in a steer-by-wire system, steering cannot be accomplished through a mechanical link.
An approach for attempting to solve the problem of loss of steering control due to the failure of the steer-by-wire system is to use the braking actuators to apply appropriate forces on the vehicle to steer the vehicle. This approach is illustrated in FIG. 1. As can be seen, a vehicle 1 is shown in a first orientation 1A traveling in a straight direction of travel 10. The center of gravity of the vehicle is shown by reference numeral 2. In the situation where the steer-by-wire system of the vehicle fails, as mentioned above, and the driver of the vehicle desires to turn the vehicle to the right to orientation 1B, for example, the driver can steer the vehicle by using the brakes. A brake force 3 is applied to right wheel 4 of the vehicle. The brake force 3 acts on the right wheel 4 on the longitudinal axis 5 of the wheel 4 with a lever arm 6 with respect to the centerline 7 of the vehicle. As can be understood, the force 3 acting on wheel 4 through lever arm 6 generates a yaw moment 8 on the vehicle about the vehicle""s center of gravity 2. The yaw moment 8 tends to rotate the vehicle 1 to the right and thus initiates a cornering motion of the vehicle to orient the vehicle in new orientation 1B. Thus, the braking force 3 applied to right wheel 4 turns the vehicle to the right.
There are disadvantages to the approach of using the brakes to steer the vehicle in a failure condition of the steer-by-wire system. For example, the cornering performance of the vehicle is limited due to the fact that the cornering forces rely solely on the longitudinal forces at the tires. Additionally, when the steering system has failed with the tires turned to a certain angle and the steering system is locked in this failed condition, the limited cornering potential that is available to the driver from use of the brakes to steer the vehicle may make it impossible to compensate completely for the locked steering system.
Therefore, it would be desirable to provide an improved apparatus and method for steering a vehicle in a failed condition for a steer-by-wire system where the driver experiences a loss of steering control due to the failure of the steer-by-wire system.
In an embodiment of the present invention, an apparatus for steering a vehicle is provided. The apparatus includes a back-drivable steer-by-wire system including a road wheel actuator assembly coupled to a wheel of the vehicle. The road wheel actuator assembly defines a steering axis and the steering axis is off-set from a longitudinal axis of the wheel by a positive scrub radius.
In an embodiment for a method in accordance with the present invention, a method to steer a vehicle after failure of a road wheel actuator assembly in a back-drivable steer-by-wire steering system is provided. The method includes the acts of applying a braking force to the wheel and generating a torque on the road wheel actuator assembly by the applied braking force acting through a positive scrub radius.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.